Fibrous materials have long been used in filter media to remove particulates from an air stream. Manufacturers of filter media have a multitude of variables to consider in choosing raw materials for their process in a specific media design. These include material properties such as polymer type, density, thickness, weight, moisture regain, and luster, as well as specific fiber characteristics such as crimp, tensile strength, crystallinity, diameter, cross-sectional shape, length, and finish.
The first step in preparing the filter media is converting the fibrous materials into a nonwoven. Nonwovens are fabric-like materials that can be made in dry laid processes by the direct melt spinning of fibrous materials collected into a web such as a spunbond process, or by processing short fiber lengths through fiber opening, blending, and consolidation into a fibrous web as in a carding process. Carding can be described as a mechanical process of individualizing and intermixing finite fibrous lengths to produce a continuous fibrous web. Once the fibrous web has been formed from carding, a secondary process of bonding is necessary to give the fibrous web integrity and strength. This bonding process may be accomplished through chemical, thermal or mechanical means.
During the carding process, fibrous materials are subjected to high stresses due to friction between fibers and metal surfaces as well as fiber-to-fiber. Desirable qualities during the carding operation include low fiber-to-metal friction, in order to reduce fiber damage, and simultaneous high fiber-to-fiber friction to promote better web cohesion, a necessary property to aid in downstream web processing and ultimate web strength. Static charges develop during carding from the transfer of electrons from one material type to another as they contact each other. This may cause the fibrous material to cling to the metal card surfaces or even repel each other, ending up in a non-uniform web of fibers having thick and thin spots. It is desirable to minimize static charges during carding in order to achieve good processing and good web uniformity necessary for producing filter media.
Carding of all fibers typically includes the use of a processing aid in order to minimize many of the aforementioned problems. The processing aid may be described as a complex, proprietary mixture of chemicals blended to specifically affect the undesired conditions mentioned above. This chemical blend is generally a well-kept trade secret for those who supply the individual ingredients as well as those who manufacture the chemical mix. This mix may include but is not limited to lubricants, emulsifiers, antistats, antimicrobial agents, cohesive agents, and wetting agents. Often the chemistry of the mix is formulated such that the components perform several different simultaneous functions.
The chemical mix used in carding of fibers is often referred to as a spin finish, and is typically applied by the fiber manufacturer. Application of the chemical mix may be performed in the melt spinning operation and is usually accomplished by a kiss roll or metered flow applicator, where typically an aqueous emulsion of the chemical mix is applied per unit weight of the fiber.
Although aqueous emulsions are desirably used in the chemical mix, other organic liquids such as alcohols or blends of organic liquids have been used. This solvent is typically removed from the chemical mix during later processing, while the primary components of the mix remain on the fibers to aid in fiber processing.
The chemical mix may also be added during the operations of drawing, crimping and cutting during manufacturing the fibrous materials. The foregoing operations ensure sufficient amount of the spin finish has been applied to the fibrous materials for processing into a fibrous nonwoven media. The spin finish level may be determined by typical extraction procedures.
Changes in spin finishes within a fiber manufacturer's facility are generally not desirable due to the down time required for cleaning and change overs, therefore it is a reasonable goal for a fiber supplier to limit the number of spin finishes within their manufacturing facility. The spin finish supplier and fiber producer rarely disclose the ingredients used in the chemical mix unless problems arise at the customer's facility like discoloration or in processing of the fibers such as carding. Even then specific details of formulations and exact chemical components are rarely fully disclosed.
Fibrous nonwoven media once produced from a carding process must then be converted to a filter. This may involve a pleating process depending upon filter design, where the media is folded upon itself to create pleats that serve to increase surface area within a given filter design. Heating may also be used in this process to aid in the folding process and setting the pleat. It is important to understand the effect that this process may have on the chemical finish that was applied in the carding process.
Nonwoven producers of fibrous filter media will use ASHRAE (American Society for Heating, Refrigerating and Air-Conditioning Engineers) Test Standard 52.2 to obtain data relating to the efficiency of the filter media. ASHRAE uses the MERV (Minimum Efficiency Rating Value) scale as a means of comparing initial efficiency ratings of various filter media. In general the higher the MERV value the higher the efficiency of the filter media. It is also common to compare efficiency values in groups depending upon particle sizes within the air stream during testing. E3, E2 and E1 values refer to particulate efficiency values at 3-10 microns, 1-3 microns and 0.3-1 microns respectively.
Until now, the art of improving the filtration efficiency of fibrous nonwoven filter media using chemical treatments has been to apply the chemical treatments to the media after the filter media has been formed. Examples include but are not limited to spraying, incorporation into the chemical binder, dipping, kiss coating, foaming and a number of other operations similar to those found in textile and a host of nonwoven operations for applying chemical treatments. Such methods add a level of complexity to the nonwoven process and often times require additional unit operations like drying to remove moisture added as a result of the treatment process.
Filtration media producers who produce filter media from fibrous materials in a carding operation have failed to consider the effect that the chemical treatment applied by the fiber producer may have on the efficiency of the media once converted into a filter. The spin finish, applied to the fibers during processing has generally been considered only as a processing aid in producing the nonwoven media. What is needed, therefore, is a method of application of chemical treatments for filter media that does not add additional complexity or additional processing steps.
In accordance with the foregoing, an embodiment of the disclosure provides a method for improving air filtration efficiency of a fibrous nonwoven media and a fibrous non-woven air filtration media having improved filtration efficiency. The method includes applying to fibrous material at least about 0.05 wt. % based on a total weight of the fibrous material of chemical treatment comprising an alkoxylated silicone copolymer. The chemical treatment is effective to provide an improvement in an E3 filtration efficiency value of the filtration media as tested according to ASHRAE 52.2 of about 20% or more compared to an E3 filtration efficiency value of said fibrous material devoid of any chemical treatment
In another embodiment, there is provided a fibrous non-woven air filtration media comprising fibrous material that includes from about 0.1 to about 0.5 wt. % based on a total weight of the fibrous material of a chemical treatment. The chemical treatment includes an alkoxylated silicone copolymer, whereby the non-woven air filtration media has an improvement in an E3 filtration efficiency value as tested according to ASHRAE 52.2 of about 20% or more compared to an E3 filtration efficiency value of fibrous material devoid of any chemical treatment.
It has been found quite surprisingly that the chemical treatment applied by the fiber producer may be used difunctionally, both as a processing aid to reduce friction and stresses in the carding operation and as a treatment to improve the particle filtration efficiency of the filter produced from such a process. By incorporating certain chemical treatments, at certain treat rates as a spin finish during the staple fiber manufacturing process, the chemical treatment may act difunctionally to aid in processing during carding during the manufacturing of the fibrous nonwoven filter media as well as to provide improvements in the filtration efficiency of the filter media when tested per ASHRAE 52.2.
Although the method of using certain chemical treatments difunctionally as a spin finish and aid to increase filtration efficiency, chemical treatment may also be applied by traditional application methods to the nonwoven web itself to provide improvements to the filtration efficiency of a filter made from the treated nonwoven web.